Swift 为分类增加属性objc_getAssociated

OC 获取关联对象

- (UIView*)backgroundView
{
    return objc_getAssociatedObject(self, &kBackgroundViewKey);
}

Swift 获取关联对象——错误的写法

func backgroundView() -> UIView
{
    return objc_getAssociatedObject(self, kBackgroundViewKey) as! UIView
}

Swift 获取关联对象——正确的写法

func backgroundView() -> UIView?
{
    return objc_getAssociatedObject(self, kBackgroundViewKey) as? UIView
}

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设置关联对象 OC 与 Swift 什么区别

OC 设置关联对象

- (void)setBackgroundView:(UIView*)backgroundView
{
    objc_setAssociatedObject(self, &kBackgroundViewKey, backgroundView, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

Swift 设置关联对象

// 这里的UIView可以是可选类型，也可以不是可选类型
func setBackgroundView(backgroundView:UIView?)
{
    objc_setAssociatedObject(self, kBackgroundViewKey, backgroundView, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
}

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